Chiral modification of copper exchanged zeolite-Y with cinchonidine and its application in the asymmetric Henry reaction

Abstract

Chirally modified Cu²⁺ exchanged zeolite-Y was synthesized by direct adsorption of cinchonidine under ambient conditions. The chirally modified materials were characterized using various spectrochemical and physicochemical techniques viz. BET, FTIR, MAS (¹H and ¹³C NMR), XPS, SEM, cyclic voltammetry and PXRD. Characteristic peaks of cinchonidine observed in the supported materials confirmed the adsorption of cinchonidine and its coordination with the Cu²⁺ active site on copper exchanged zeolite-Y. ¹³C SSNMR and XPS analysis however confirmed for the half encapsulation process, only the quinoline ring of cinchonidine gets coordinated to the internal metal sites via the N atom while the quinuclidine moiety extends out of the host surface. Cinchonidine supported Cu²⁺-Y zeolites were found to exhibit good catalytic performance in the asymmetric Henry reaction. ¹H SSNMR studies also confirmed the protonation of the N atom of the quinuclidine ring during the course of the Henry reaction. Heterogeneous chiral catalysts were effective for up to two consecutive cycles. Leaching of cinchonidine after the second cycle was found to have a negative result in the catalytic performance.